The invention relates to apparatus for measuring stiffness of a fractured bone in the course of bone healing, wherein the healing has been aided by an external fixator for retention of the broken segments.
To date most of the clinical data and research on bone-fracture healing with external fixation have been concerned with measurements on the tibia, because of the frequency of tibia fracture and the need to restore weight-bearing capability at the repaired site. Thus, in the present discussion, the tibia will be considered illustratively, since principles applicable to the tibia are applicable to other bones in the course of fracture repair.
The paper entitled: "The Measurement of Stiffness of Fractures Treated With External Fixation", Cunningham, et al., Engineering in Medicine, Vol. 16, No. 4, 1987, describes apparatus and a technique for indirectly measuring fracture stiffness, periodically in the course of healing a fractured tibia, wherein the patient is seated and rests the heel of his broken limb on a load cell so that the fractured bone, including its external fixator, are otherwise unsupported. The fixator is equipped with a transducer which is able to measure bending as a function of vertically downward force application to the leg. This technique has the disadvantage that even if bone-screw anchorages retain their fidelity, the deflection measurement must include a correctional calculation which reflects the fact that primary stiffness is in the fixator. Bone-stiffness measurement is thus indirect, and as a practical matter, the need to stress the bone in order to stress the fixator is the occasion for progressive deterioration of the effectiveness of bone-screw anchorage, resulting in progressive loss of measurement accuracy. To forestall the loss of bone-screw anchorage, one must severely limit the number and frequency of such measurements.
Published European Patent Application A0,324,279 describes apparatus for direct measurement of bone stiffness in circumstances generally similar to those of the Cunningham, et al. article, except that for purposes of making the deflection measurement, the fixator used for aiding fracture repair is temporarily removed, leaving fixator bone screws in place; and brackets releasably clamped to the bone screws provide proximal and distal points of support for the respective end mounts of a flexible elongate goniometer. The goniometer is the only external connection between the bone stubs on longitudinally opposite sides of the fracture, and therefore deflections measured for the vertically downward force applied to the limb at the fracture site are direct measurements, requiring no compensating calculation for fixator or goniometer stiffness, because the goniometer structure is inherently flexible and limp, and of negligible stiffness.
Despite the potential for direct measurement afforded by the structure of said published European patent application, the apparatus is structurally relatively crude, and therefore repeatability of measurements at any given occasion is somewhat open to chance, particularly in respect of desired alignment and orientation of the mounting ends of the goniometer, with respect to each other and with respect to the fractured bone.